This present invention relates to a process for the manufacture and use of a polypropylene film (PP film) with improved adhesion properties.
For many areas of use, the properties of materials must be improved by treating the surface, for example, by lacquering it or coating it with thin finishing films. At the present time, PVC and special papers are chiefly used for improving the surface of materials, especially of metallic, wooden, and cellulose materials.
In using films in various areas of surface improvement, one should proceed from the fact that the coating should take place on carrier materials and other films at temeratures that are frequently above 100.degree. C., and cooling the laminates in the roller coating mills and presses is not economically practical. A higher resistance to heat in retaining shape is also necessary in coating with reaction adhesives which, because of the required stability in storage, should properly react only above 80.degree. C. It is also required in coating with polymer films, that no delamination is to take place in the range of the working temperature.
As far as the physical properties of PP and its copolymerizates are concerned, especially resistance to heat in retaining shape, as well as its environmental safety, PP would by very suitable for improving surface. A decisive disadvantage, however, are the surface energy materials characteristics of PP, which prevent sufficiently good, permanent adhesion of suitable lacquers, adhesives, and polymer films.
The basis of the present invention is to improved the adhesion properties of PP Films by permanently raising the surface energy materials characteristics in such a way that suitable printing, lacquering, and adhesion become possible in a permanent manner.
Processes for the improvement of the adhesion properties of films, especially PP films, are well known. The Corona treatment is an example of this; cf. Adhasion, Vol. 12 (1979) pp. 381-389. This treatment, to be sure, temporarily increases surface tension; however, the effect decreases during storage and with higher temperatures.
Another process consists of the flame treatment of films. Here the same disadvantages prevail as in the Corona treatment. Chemical treatment with ozone, fluorine, chlorine, etc. is also well known. The problematic use, particularly, is disadvantageous here, because of the chemical reactivity of these agents. Another process consists of radiation with energized rays, such as, electron rays, UV rays, laser beams, and the like. These processes are relatively expensive, usually require the concomitant use of sensitizers, and at the same time, the permanence of the effect varies.
Permanent improvement of surface adhesion can be achieved by chemical grafting with the use of energized rays. This process, however, is too expensive to be suitable for the area of use under consideration.